Process of heat-treating and hardening projectiles



units stares GFFECB JOHN L. COX, OF PHILADELPHIA, PENNSYLVANIA.

PROCESS OF HEAT-TREATING AND HARDENING PROJECTILES.

amas.

Ho Drawing. Application filed July 16, 1919,

To all whom it may concern:

Be it known that 1, JOHN L. Cox, a citizen of the United States,residing at Philadel- Ehia, county of Philadelphia, and State ofennsylvania, have invented a new and useful Improvement in Processes ofHeat- Treating and Hardening Projectiles, oi which the following is afull, clear, and exact description.

In the manufacture of armor piercing shot and shell the necessity ofcombining in a. projectile formed of a single block or" metal a hard butnot brittle head with a relat vely soft and very tough body is wellrecognized, and projectiles having in degree such characteristicshavehitherto been manufactured.

Known processes of manufacture still leave much to be desired inobtaining and combining these desirable and apparently contradictoryqualities; moreover, a considerable proportion of projectiles that havebeen subjected to recognized methods of heat treatment designed toproduce the desired result fail when subjected to the rigorous ballistictests which they must pass to be accepted.

It is the object of my invention to manufacture a projectile which willpossess to a maximum. degree the qual ties above mentioned, and by aprocess which Wlll mlnimlze the likelihood of producmg any defects orWeaknesses which might endanger acceptance or, later, cause spontaneousrupture in storage.

By my process a pro ectlle is produced having a point characterized by ahigh degree of hardness, progressively decreas ng through the headtoward its unction wltli the body, and a body sufiiclently soft topermil; machining yet so tough that 1t wlll not break upon impact;moreover the process is one wherein the danger of cracking duringtreatment is minimized, and all dangerof the shell cracking in storageafter delivery 1s removed.

In the heat-treatment of steel certam laws are. more or less well known,but there are many conceivable ways of applying them to heat treatingprocesses, and there is always the danger that while some of these lawsmay be regarded others might be overlooked or ignored.

The laws that I have more especially borne in mind in the development ofmy process are the following:

1. On heating, at the Ac po nt the previ- Specification of LettersPatent.

Serial No. 311,325. No. 427,232.

Patented Feb. 21, 1%222 Renewed November 29, 1920. Serial ously existingcell boundaries of the steel begm to disappear as the solutioncommences, but are not entirely gone until the Ac point is reached.

2. At the Ac point a new cell structure begins to form and to growlarger as the temperature increases.

3. lChere is a cell size corresponding roughly to any given temperatureabove the Ac point, long maintenance at that temperature tending ratherto increase the thickness of the cell boundaries than to increase thecell size.

4. To agquire, on sudden cooling, the maximum hardness permitted by itschemical composition steel must first be heated to or above the Ac,point.

5. The degree or" final hardness is dependent upon the speed of thequench.

6. Steel raised to a. temperature much in excess of the Ac, point tendsto become coarse in grain and brittle.

7. lhe power of hardening once acquired by steel on heating is not lost,on slow cooling, until the Ar, point is passed.

8. Sudden cooling from below the Ar point theoretically will not producehardenmg but the steel will be in the same soft or annealed state thatit would be had it-been slowly cooled.

It is much less well known that on heating, sudden cooling from a pointwithin a substantial, but limited, range below the Ac point asexperimentally determined. may produce a distinct degree of hardeningand notable loss of toughness; while sudden cooling from a temperaturetoo far below the A0 point to produce this effect-will impart a degreeof toughness superior-to that resulting from a slow cooling from thesame or some what higher temperature.

It is also known to but few that the Ac and Ac points are not fixedpoints for any particular composition but vary with the degree ofgrain-refinement of the steel, being lower as the grain of the steel isfiner.

I shall not herein describe the process of melting the steel andcasting, or casting and forging, the projectile.

Afterproduction, the projectile, as it comes from the forge at atemperature substantially above the Ac point, maybe cooled slowly untilbelow the Ar point; but preferably,.in order to arrest the growth of thecell walls,

the projectile may be cooled suddenly to ap- 'oximately the Ar point,and thereafter oled more slowly. The next procedure is to refine thegrain .d soften for preparatory machining. The 'ojectile is raised to atemperature above the point, and maintained at that temperare longenough to allow complete solution the microscopic constituents. It maythen cooled with extreme slowness, to below e Ar point; but preferably,in order to rein as fine a grain as possible, it is cooled .ickly tobelow the Ar point, then reated to below the Ac point, maintained at attemperature for a time, and allowed to 01 slowly or rapidly. Theprojectile is now in the best possible udition for furtherheat-treatment, and is, )reqver, in an extremely soft condition per.tting easy preparatory machining. After the preparatory machining thepro- :tile is then still further grain-refined. It heated to about 130C. above the Ac, int, maintained there a proper length of ac to insurecomplete solution,-the time pending upon its size-is rapidly withawnfrom the furnace and quenched in until well below the Ar point throughtits entire mass. Depending upon the calibre of the projec- 2, this stepis applied one or twice. The )jectile is next heated to a temperature ofout C. above the Ac, point, similarly Lintained at that temperature andsimily quenched. [t is to be observed that in both cases the aperaturesare higher than the theoretily correct points but are given in prace tohasten the necessary molecular trans- -mations, while the time is not tobe long )ugh to cause appreciable grain growth. lhe projectile is nextheated to a temperae about 75 C. below the Ac point and inched for a fewminutes in warm water, necessary care being taken to remove the )jectilefrom the water before cracking urs. The projectile is then allowed to lto atmospheric temperature. f the composition of the projectile has nproperly chosen and all steps of meltforging, and the heat-treatmentdeibed above properly applied, the projecwill be in the condition ofmaximum ghness throughout, and soft enough read to allow machining tofinish sizes, should t step not already have been applied. It generallyadvisable not to finish the band re at this stage. r The projectilebeing ready for its final dening I first preliminarily heat it in a naceto a temperature below the Ac at, and then heat the head of theprojecand at least the forward part of the y, to above the Ac point ofthe steel. s is best done by holding the projectile its'base andimmersing the head and all or nearly all of the body in a bath of meltedlead raised to the necessary temperature. Bearing in mind that the headis solid and body hollow, and that, therefore, the greater thickness ofmetal is in the head, it is best at first to immerse only the point andhead, or (say) that, part of the projectile forward of the tip of thecavity, and, after they have acquired substantially the temperature ofthe lead then to submerge all or nearly all of the body.

If the projectile be held by a mechanism operating in the bore it shouldbe possible to submerge practically the entire body. If the handlingmechanism holds the projectile by the exterior near the base it shouldbe possible to submerge the projectile to about the line of the bandscore.

After the projectile has been heated up to the prescribed temperatureand been maintained at that temperature for a sufficient length of timefor the full development of its capacity for hardening it is withdrawnand quenched in a suitable liquid or liquids, for which purpose I preferto employ a spray of water under pressure. The spraying of the liquid iscontinued until the temperature of the projectile approaches the normal,say below 120 C.

As soon after the hardening as possible, I slowly warm the projectile,conveniently by immersion in hot water, and after it has acquired thetemperature of the water, transfer it to a bath of heated oil in whichit is kept for alength of time suflicient for the practical completionof the drawing effects of that temperature. The exact temperature andtime of the drawing depend somewhatupon the composition and calibre ofthe projectile, the first being not over 200 and generally not over 150C. nor under (3., and the latter generally not less than six hours.

By this step the mineralogical hardness of the projectile is veryslightly reduced but the loss in hardness is far more than offset by theincrease in toughness of the slightly tempered steel and'by the releasein large part of the stresses set up in hardening.

roperly applied, this step has proved a complete cure for the'seriouslosses which formerly occurred through spontaneous rupture of finishedprojectiles,-even months after their delivery.

After cleaning the projectile from oil I then heat the body, from thebase end, to about 70 C. below the Ac point of the steel. This is bestdone by holding the projectile by the head and immersing the body,either gradually or suddenly, in a bath of lead heated to the necessarytemperature. The depth of this immersion and the temperature of the leadwill vary somewhat with the precise results that are sought to beobtained. Generally, it is best first to immerse the projectile to aboutthe rear edge of the band score,

thoroughly draw the hardness from that portion of the projectile. It maysometimes be then further immersed for a shorter period of time as faras the forward edge of the bourrelet and there maintained until thehardness of the head remaining outside of the lead has been draw tonearly the desired degree.

I then remove the projectile from the lead and quench the body in asprayof water under pressure, care being taken, preferably, to

leave a narrow zone of incandescent metal above the line of the spray,and therefore unquenched by. it, to avoid danger of cracking the head.

As soon as the body of the projectile has reached normal temperature,which, in such a spray as I have described, will occur in a very fewminutes, I invert the projectile and place it point downwards in ashallow bath of. water to prevent the further softening of the point,which would otherwise occur through the transmission of heat from theheavier section of the head.

The effect of the above treatments is to impart to the head the maximumdegree of hardness at the point, and a diminishing degree of hardnesstowards the rear without impairing the fine-grained structure of thesteel, while imparting to the body the submerge the point in water,thereb maximum degree of toughness, and a suflicient degree of softnessto enable it to be readily machined if desired, as for instance, tor theattachment of the band.

While it is true that the elevation of the temperature of the rear ofthe head in the final heating operation due to the conductivity of themetal modifies somewhat its hardness, the modification is such that itwould be desirable to take special means to obtain it; that is, theobjectionable brittleness necessarily associated with extreme hardnessis largely eliminated where maximum hardness is not necessary, but thereis secured a maximum vdegree of hardness consistent with the reductionof brittleness to the desired extent. However, the amount of heattransmitted from the head to the point during the cooling of the headmust be restricted to prevent diminution of the hardness of the point.

As it is usually dangerous to quench the whole projectile after theheating of the body, in that the projectile is likely to crack, Iprefer, as described above, to quench sub stantially all the body andsubsequentlllylgo 0 ing down the temperature of the point.

I wish it to be understood that there is no clear line of demarcationbetween the hard and tough portions of the projectile;

on the contrary there is between the hard tion of the head, shading inhardness from the (point toward the body, which is not as har as thepoint nor as tough as the major portion of the body. 7

While I have referred to the projectiles to which my process is appliedas steel projectiles, it is not intended that the process shall belimited to projectiles consisting of simple carbon steel only. I mean toinclude projectiles of steel alloyed with other metals,variouscompositions of said alloys being well known and successfully used.

The exact temperatures corresponding to theAc Ac and Ar points will varywith the composition and physical structure of the steel and cannot bedefinitely stated; but these points are well known to metallurgists andpractical steel workers and are more informative than the specificationof exact temperatures. Instruments of a number of forms exist for theiraccurate determination.

Where, in the specification, I use the term slow cooling, I mean toimply that the rate of cooling, as compared with free cooling in air, isretarded. "Where quick cooling is specified, I mean to include not onlyquenching in liquid, but also freely cooling in air, the latteroperation bein rapid relatively to retarded cooling. y ,sudden coolingor quenching I mean to imply the use of a liquid to accelerate cooling.

Having now fully described my invention, what I claim and desire toprotect by Letters Patent is:

1. The process of heat treating projectiles which comprises suddenlycooling the projectile from a temperature substantially above the A0point to a temperature not higher than approximately the A r point, thusarresting the growth of the cell walls, then cooling relatively slowly,then heating to and above the Ac, point, then cooling quickly to belowthe Ar point, then heating to below-the Ac point, then cooling, thensubjecting the projectile to a plurality of gram-refining operations inone of which the projectile is heated to substantially above the A0,,point and cooled suddenly to below the Ar, oint and in a later one ofwhich the pro ectile is heated to -a lower temperature but still abovethe Ac point and similarly cooled below the Ar point, then heating tobelow the Ac oint and quenching and cooling to atmosp eric temperature,then heating the head and at least the forward part of the body to abovethe Ac point, suddenly cooling by quenching, drawing the temper of thehardened part of the projectile at a temperature below 200 (3., heatingthe body of the projectile from the base end to below the Ac point,quenching the last named part of the pro- :tile, and then placing thepoint of the ojectile in a cooling medium to preserve e desired degreeof hardness of the point. 2. The process of heat treating projeces whichcomprises suddenly cooling the ojectile from a temperature substantiallyove the Ac point to a temperature not gher than approximately the Ar,point, us arresting the growth of the cell walls, en cooling relativelyslowly, then heating above the Ac point, then cooling quickly below theAr, point, then heating to low the Ac, point, then cooling, thensubcting the projectile to a plurality of ain-refining operations in oneof which e projectile is heated to substantially rovethe' Ac point andcooled suddenly to :low the Ar, point and in a later one of hich theprojectile is heated to a lower mperature but still above the Ac point1d similarly cooled below the Ar, point, .en heating to below the Acpoint and ienching and cooling to atmospheric temzrature, and thensubjecting the projectile a hardening treatment.

3. The process of heat treating projectiles hich comprises initiallysuddenly cooling .e projectile from a temperature substan- -ally abovethe Ac, point to a temperature it higher than approximately the Ar,)int, thus arresting the growth of the cell alls, and thereafter coolingrelatively owly, then-subjecting the projectile to sucssive heating andcooling to refine the grain 1d soften for preparatory machining, thenLbjecting the projectile to a succession of :ating andcooling treatmentsto further fine the grain and impart to the projectile 1e toughcondition known as fibrous, and [en subjecting the projectile to a finalirdenirig treatment.

4. The process of heat treating projectiles hich comprises initiallysuddenly cooling [8 projectile from a temperature substanally above theAc point to a temperature )t higher than approximately the Ar, point[11S arresting the growth of the cell walls, 1d thereafter coolingrelatively slowly, nd then refining the grain and softening )rpreparatory machining by heating the rojectile to above the Ac point ofthe steel, ien cooling quickly to not approximately bove the Ar, point,then reheating to below 1e Ac, point and then cooling.

The process of heat treating projectiles 'hic'h comprises first refiningthe grain and nparting a tough fibrous condition to the zeel bysubjecting the projectile to heating nd cooling to refine the grain andsoften or preparatory machining, then heating to bove the Ac point andquenching, heating 3 below the Ac, point and again quenching; ndafterward subjecting the projectile to a ardening treatment.

6. The process of heat treating pro ectiles preparatory to finalhardening which com prises heating the projectile above the Ac point anduickly cooling to a temperature below the r, point, reheating to belowthe Ac, point and cooling, thereby refining the grain and softening forpreparatory machining, heating to above the Ac point and quenching tobelow the Ar, point, and heating to below the Ac, point and quenching,thereby imparting a tough fibrous condition to the steel. I

7 The process of heat treating projectiles which comprises subjectingthe projectile to heating and cooling to soften forpreparatory machiningand then subjecting the projectile to a plurality of grain-refiningoperations in one of which the projectile is heated to substantiallyabove the Ac point and cooled suddenly well below the Ar, point and in alater one of which the projectile is heated to a lower temperature butstill above the Ac, point and similarly suddenly cooled below the Ar,point. 1

8. The process of hardening heat-treated projectiles which comprisesheating the head and at least the forward part of the body to above theAc point of the steel, quenching the projectile in a suitable coolingmedium so as to produce maximum hardness of the steel, heating the bodyof the projectile from the base end to below the Ac, point of the steel,while maintaining the point out of contact with the heating medium, andquenching only the body of the projectile in a suitable cooling mediumso as to produce maximum toughness of the body.

9. The process of hardening heat-treated projectiles which comprisesheating the head and at least the forward part of the body to above theAc point of the steel, suddenly coolin heating the body of theprojectile to be ow the Ac, point of the steel and suddenly cooling thebody, and, after substantial cooling of the body has occurred, bringingand holding the point of the projectile under the influence of a coolingmedium until the whole projectile is cooled.

10. The process of hardening heat-treated projectiles which comprisesheating the head and at least the forward'portion of the body to abovethe Ac, point of the steel while holding the projectile by the base,quenching the projectile in a suitable cooling medium, heating the bodyof the projectile to below the Ac, point of the steel while holding therojectile by the head and maintaming tie point out of contact with theheating medium, and quenching only the body in a suitable coolingmedium.

11. The process of hardening heat-treated projectiles which comprisesheating the head i and at least the forward part of thebody to above theAc point, suddenly cooling, drawing the temper of the hardened part ofthe projectile at a temperature below 200 (3., heating the body of theprojectile from the base end to below the Ac point, and quenching thelast named part ofthe projectile.

12. The process of hardening heat treated projectiles which comprisesheating the head and at least the forward part of the body to above theA0 point of the steel, quenching the projectile in a. suitable coolingmedium so as to produce maximum hardness of the steel, immersing a partof the projectile from its base end in a heating medium, then after aninterval immersing the projectile in said heating medium to a greaterdepth While maintaining the point out of contact with the heatingmedium, and then quenching the larger part of the heated portion of theprojectile from the base forward while maintaining the point unquenched.

13. The process of heat treating projectiles which comprises heating theprojectile to above the A0 point, then cooling,,afterward heating toabove the Ac point, then cooling suddenly, then heating to below the Acpoint, then cooling, then heating the head and at least the forward partof the body to above the A0 point, suddenly cooling by quenching,heating the body of th' projectile from the base end to below theAcpoint, and afterward placing the point of tilt projectile in a coolingmedium to preservi the desired degree of hardness of the point 14. Theprocess of hardening heat treatec projectiles which comprises heatingthe head and at least the forward part of the body tc above the Ac pointof the steel, quenching the projectile in a suitable cooling medium soas to produce maximum hardness of the steel, immersing a part of theprojectile from its base end in a heating medium, then after an intervalimmersing the projectile in said heating medium to a greater depth whilemaintaining the point out of contact with the heating medium, and thenquenching the larger part of the heated portion of the projectile fromthe base forward while maintaining above the cooling medium a narrowzone of incandescent metal to avoid cracking of the head.

In testimony of which invention, I have hereunto set my hand, atPhiladelphia, Pa, on this 1st day of July, 1919.

JOHN L. COX.

